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RE: Cluster satellites
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Cluster: ESA spacecraft flying closer than ever for better science
After weeks of manoeuvres, Samba and Tango, two of ESAs four Cluster satellites are now orbiting in formation, separated by only 17 km. This is the closest two ESA satellites have ever been in routine operations and will enable new scientific discoveries.
 Cluster, ESAs mission comprising four identical satellites, relays the most detailed information ever about how the solar wind affects our planet, and is the first mission to study the Sun-Earth connection in 3D.
This is done by studying the behaviour of near-Earth plasma, an extremely viable gas, composed of ions and electrons but electrically neutral, spread over large distances. A key to understanding it and studying complex geophysical processes in different regions is to have space-based, multi-point observations and to be able to vary the distances between spacecraft, as these processes operate at different scales in nature.

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ESAs Cluster was in the right place and time to make a shocking discovery. The four spacecraft encountered a shock wave that kept breaking and reforming predicted only in theory.
 On 24 January 2001, Clusters spacecraft observed shock reformation in the Earths magnetosphere, predicted only in theory, over 20 years ago. Cluster provided the first opportunity ever to observe such an event, the details of which have been published in a paper on 9 March this year.
The shock wave that sits above the Earths surface is a natural phenomenon. It is located on the side facing the Sun, at approximately one quarter of the distance to the Moon, and is caused by the flow of electrically charged particles from the Sun.
This flow of electrically charged particles known as solar wind is emitted in a gusty manner by the Sun. When it collides with the Earths magnetic field, it is abruptly slowed down and this causes a barrier of electrified gas, called the bow shock, to build up. It behaves in the same way as water being pushed out of the way by the front of a ship.
On 24 January 2001, the four Cluster spacecraft were flying at an approximate altitude of 105 000 kilometres, in tetrahedron formation. Each spacecraft was separated from the others by a distance of about 600 kilometres. With such a distance between them, as they approached the bow shock, scientists expected that every spacecraft would record a similar signature of the passage through this region.

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Substorms
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Cluster is providing new insights into the working of a space tsunami that plays a role in disrupting the calm and beautiful aurora, or northern lights, creating patterns of auroral dances in the sky.
 Generally seen in high-latitude regions such as Scandinavia or Canada, aurorae are colourful curtains of light that appear in the sky. Caused by the interaction of high-energy particles brought by the solar wind with Earths magnetic field, they appear in many different shapes.  
Early in the evening, the aurora often forms a motionless green arc that stretches across the sky in the east-west direction. Colourful dancing auroral forms are the results of disturbances known as substorms taking place in Earths magnetosphere. These perturbations can affect our daily lives, in particular by affecting the reception of GPS signals. Thus, understanding the physical processes involved is important to our routine life and security.
These substorms typically last one to two hours and are three-dimensional physical phenomena spread over altitudes from 100 to 150 000 kilometres. Trying to understand such complex physical processes with a single scientific spacecraft is like trying to predict the behaviour of a tsunami with a single buoy in an ocean. That is why the simultaneous use of several satellites, like the Cluster constellation, is necessary to understand these events.

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Cluster satellites
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Using measurements of the four ESA's Cluster satellites, a study published this week in Nature Physics shows pioneering experimental evidence of magnetic reconnection also in turbulent 'plasma' around Earth.
Magnetic reconnection – a phenomenon by which magnetic fields lines get interconnected and reconfigure themselves - is a universal process in space that plays a key role in various astrophysical phenomena such as star formation, solar explosions or the entry of solar material within the Earth's environment. Reconnection has been observed at large-scale boundaries between different plasma environments such as the boundary between Earth and interplanetary space. Plasma is a gas composed of charged particles.

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Magnetic whirlpools feed Earth’s magnetosphere
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Giant whirlpools of electrically charged gas, some 40 000 kilometres across, have been witnessed above the Earth by a team of European and American scientists. Using data from ESA's Cluster quartet of spacecraft, the researchers have shown that these whirlpools inject electrified gas into the magnetic environment of the Earth.
The magnetic field generated inside the Earth protects the planet from the electrically charged particles given out by the Sun. However, it is only a partially effective shield.
In the same way that a car can only travel along roads, electrically charged gas, known as plasma, can only travel along magnetic field lines, never across them. For a car to suddenly change direction, it has to use a road junction.
For a particle of plasma to suddenly jump onto a different field line, there has to be a reconnection event. In a reconnection, magnetic fields lines spontaneously break and then join up with other nearby lines. In doing so, the plasma is suddenly redirected along new routes.
Scientists know this happens in Earth's magnetosphere because of changes to the plasma sheet. This is one of the inner regions of the Earth's magnetic field. Plasma in the sheet is usually hot and tenuous, whereas the solar wind is cooler and denser. At certain times, the sheet fills with cooler and denser plasma over the course of a few hours.
For this to happen, the solar wind plasma must somehow be able to cross the Earth’s magnetic boundary, known as the magnetopause. Yet, until now scientists had no observational evidence that reconnection inside whirlpools contribute to this process.

"Wondering how the solar wind could get into the plasma sheet is how I became interested in this problem" - Katariina Nykyri, lead author of the results, from the Imperial College, London, UK.

Together with colleagues, Nykyri began investigating a strange event recorded by Cluster on 3 July 2001. At this time, Cluster was passing the dawn side of the Earth. In this region of space, the solar wind is sliding past the Earth's magnetopause, in roughly the same way as wind blows across the surface of an ocean.
A previous Cluster observation had shown that whirlpools of plasma can be whipped up by this configuration. Such whirlpools are known as Kelvin-Helmholtz instabilities and scientists suspected them of being the location of magnetic reconnection but no one had found any conclusive evidence that this was the case.
Cluster can recognise magnetic reconnection because of what researchers call 'rotational discontinuities'. These show up as sudden changes in the direction of the plasma flow and magnetic field. After a painstaking analysis, Nykyri found just such rotational discontinuities in the data for 3 July 2001. To be certain of her result, she reanalysed the data four times.
Then she developed a computer model to simulate the event. The computer showed that the Cluster data was only understandable if the whirlpools were causing magnetic reconnection to take place. In these reconnections, plasma was being fed down through the magnetic boundary of Earth and into the magnetosphere.
The work does not stop there. Nykyri and colleagues are now developing more sophisticated computer simulations to understand the whirlpools’ three-dimensional behaviour.

"This is a very big challenge," she says, because of the additional numerical processes involved.

She also plans to search the Cluster data archive for more examples of these events.

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Source of Mysterious High Speed Electrons
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A team of scientists led by University of Maryland physics professor James Drake appear to have solved a key remaining mystery about how the interaction of magnetic fields produce the explosive releases of energy seen in solar flares, storms in the Earth's magnetosphere and many other powerful cosmic events.

In recent years, researchers have answered many questions about this process, but one thing they have been unable to explain is data from solar observing satellites indicating that up to half of the energy released during solar flares is in the form of energetic electrons. Large numbers of these low-mass particles travel at speeds far higher than can be explained by the widely accepted "slingshot" model for how reconnecting magnetic field lines accelerate charged particles.

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RE: Cluster satellites
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On 15 September, flight controllers at ESA's Space Operations Centre watched tensely as 'Rumba', No. 1 in the four-spacecraft Cluster fleet, was switched into a low-power, deep hibernation mode. The aim was to survive a challenging eclipse.

Each year, in autumn, the Cluster fleet must pass several times through the Earth's shadow with respect to the Sun. During these eclipses, which last about three hours, sunlight is blocked by the Earth and the spacecraft solar panels cannot generate electricity.
Batteries then generate electrical power, which is used to run heaters - keeping the spacecraft warm - and to operate on-board computers and maintain spacecraft control.
However, the Cluster fleet, launched in mid-2000, has been in orbit for over six years and batteries, in particular, are beginning to age. This year, spacecraft No. 1, Rumba, had the most severe battery problems; three out of five have already been declared non-operational for nominal use and one has a high leakage current.
As a result, mission engineers forecast that only about half of the battery power required by Rumba would be available during eclipse season.

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Details of solar particles penetrating the Earth’s environment revealed
Co-ordinated efforts by China/ESA’s Double Star and ESA’s Cluster spacecraft have allowed scientists to zero in on an area where energetic particles from the Sun are blasting their way through the Earth’s magnetic shield. Solar material penetrating the Earth's magnetic shield can represent a hazard to both astronauts and satellites.
On 8 May 2004, one of the two Double Star satellites (TC-1) and all four Cluster spacecraft found themselves in the firing line. For about 6 hours, the Cluster spacecraft were buffeted every 8 minutes by intense flows of electrically charged particles released by the Sun. The Double Star TC-1 spacecraft had it even rougher, being blasted every four minutes for eight hours.

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Double Star and Cluster witness pulsated reconnection for several hours
Using coordinated observations of the CNSA/ESA Double Star and ESA/NASA Cluster missions, a team of European and US scientists reveals new features of magnetic reconnection at the Earth's magnetopause. These results improve our knowledge on how, where and under which conditions the solar wind manages to penetrate the Earth's magnetic shield on the flank of the magnetosphere.

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Bursty bulk flows
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Auroral displays over Canada pictured from the International Space Station from an altitude of 400 kilometres. The Manicouagan impact crater is visible in the foreground.


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Credits: NASA

Thanks to observations performed in 2001 and 2002, ESA’s Cluster mission has established that high-speed flows of electrified gas, known as bursty bulk flows, in the Earth’s magnetic field are the carriers of decisive amounts of mass, energy and magnetic perturbation towards the Earth during magnetic substorms. When substorms occur, energetic particles strike our atmosphere, causing auroras to shine.

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